In EUTRAN (Evolved UTRAN), which is currently being examined as the next generation system according to the 3GPP, a handover performed between different cells respectively covered by different base stations in response to the moving of a mobile terminal station is handled as a hard handover. In order to perform a hard handover, a line that connects a mobile terminal and a base station before the moving of the mobile terminal is disconnected, and thereafter a line between the mobile terminal and another base station covering the movement destination is connected. Although a handover can be performed in a short period even when that handover is a hard handover by obtaining system information on the base station covering the movement destination immediately before performing the handover, the transmission of user data is interrupted during the handover.
In the EUTRAN, the usage of a CAZAC (Constant Amplitude Zero Auto Correlation) sequence as a sign used in the preamble portion of a data frame is dominant. The preamble portion of a data frame contains a random access signal transmitted in an uplink line. A Zadoff-Chu sequence, a GCL sequence, etc. can be used as a CAZAC sequence. An expression expressing a Zadoff-Chu sequence is given below, where L represents a sequence length, and k represents a sequence index. Particularly when the length l is a prime number, an excellent autocorrelation characteristic and a cross-correlation characteristic are obtained.
                                                                                                              c                    k                                    ⁡                                      (                    n                    )                                                  =                                  exp                  ⁡                                      [                                                                                            j2π                          ⁢                                                                                                          ⁢                          k                                                L                                            ⁢                                              (                                                  n                          +                                                      n                            ⁢                                                                                          n                                +                                1                                                            2                                                                                                      )                                                              ]                                                                                                      if                ⁢                                                                  ⁢                L                ⁢                                                                  ⁢                is                ⁢                                                                  ⁢                odd                                                    ⁢                                  ⁢                                                                                                  c                    k                                    ⁡                                      (                    n                    )                                                  =                                  exp                  ⁡                                      [                                                                                            j2π                          ⁢                                                                                                          ⁢                          k                                                L                                            ⁢                                              (                                                  n                          +                                                                                    n                              2                                                        2                                                                          )                                                              ]                                                                                                      if                ⁢                                                                  ⁢                L                ⁢                                                                  ⁢                is                ⁢                                                                  ⁢                even                                                                        [                  Expression          ⁢                                          ⁢          1                ]            
FIG. 1 is a block diagram illustrating an example of a circuit generating, from a CAZAC sequence, a preamble portion of a random access signal used in a wireless uplink according to the EUTRAN.
A CAZAC sequence with a length of M(L) is subject to a serial/parallel conversion, and is input into a DFT unit 10, and thereafter is Fourier-transformed into a parallel signal with parallel number M. This signal is input into a subcarrier mapping unit 11, and is mapped into N subcarriers. The subcarrier signals obtained by mapping the CAZAC sequence are input into an IFFT unit 12, and are subject to an inverse Fourier transform, and thereafter are input into a parallel/serial conversion unit 13. A signal with parallel number N output from the IFFT unit 12 is converted into a serial signal by the parallel/serial conversion unit 13, and is output as a random access preamble sequence.
Further, according to the EUTRAN, the maximum system bandwidth (a sending/receiving bandwidth used in a wireless transmission line by base stations) for transmission and reception in the radio section is 20 MHz, and the transmission/reception minimum bandwidth for terminal stations is currently set to be 10 MHz. This means that terminal stations are capable of transmitting and receiving signals having a bandwidth of at least 10 MHz. Also, it is considered that cells can be arranged in such a manner that base stations having different system bandwidths are adjacent to each other. In order to permit terminal stations to perform the initial search and to become a handover target easily regardless of the system bandwidths, synchronization channels and broadcast signal channels (a broadcast signal channel is a channel for transmitting information on cells and base stations, and is referred to as a broadcast channel hereinafter) are arranged around the middle of the downlink transmission bandwidth. However, it is also considered that synchronization channels can be arranged at two or three portions in a transmission bandwidth when the system transmission bandwidth is 20 MHz. This is based on the consideration of the existence of a mobile terminal station transmitting and receiving signals having a bandwidth of 10 MHz.
In any of the cases described above, synchronization channels are not always inserted into all subframes, and a synchronization channel is set to be inserted into every 5, 10 or 20 subframes to be transmitted.
According to the EUTRAN, a handover between adjacent cells covered by different base stations is handled as a hard handover, and in a hard handover, when a mobile terminal station starts a handover while it is transmitting data, a downlink transmission being performed targeting that mobile terminal station is interrupted, and the downlink data is transferred to the base station in the handover destination from the base station that has been communicating with that mobile terminal device. Also, a buffer in a base station has to accumulate the downlink data for the mobile terminal device until the handover is completed, and as a period required for performing a handover becomes longer, the amount of data that is accumulated in a buffer increases as well. Similarly, uplink data to be transmitted from the mobile terminal station to the base station has to be prevented from being transmitted when a handover has started, and has to be accumulated in a buffer in a mobile terminal station. In such a case, some data may be discarded in a process of the handover, which requires retransmission using a higher-level layer in the case of normal data. Further, when a handover has started with successive pieces of data being transmitted on the basis of the HARQ (Hybrid Automatic Repeat request), the order of sequence numbers assigned to the successive pieces of data may be changed. Also, when a handover is performed while an audio communication is being performed using speech packets, the downlink speech packets are transferred to the base station in the handover destination; however, it is impossible to avoid an interruption. Also, audio communication is real time communication, and accordingly all speech packets transferred are not always utilized effectively in the transfer destination.
Because of the above facts, it is not desirable for a handover to consume a long period of time regardless of whether data being transmitted is normal data or speech packets.
In a process of a handover, a terminal station has to capture a downlink synchronization channel transmitted from the base station in the handover destination, has to synchronize itself with the base station, and has to transmit a random access signal in an uplink channel. If a random access signal can be transmitted immediately after the completion of the downlink synchronization, time consumed by a handover can be reduced.
Also, when CAZAC sequences are used as the preamble portions of random access signals, the values of the PAPR (peak-to-average power ratio) of the preamble portions vary depending upon the values of the index k even when the length l of the CAZAC sequences used does not vary.
FIG. 2 depicts a PAPR characteristic of a preamble signal having a bandwidth of 5 MHz and using a Zadoff-Chu sequence.
The sequence length of the Zadoff-Chu sequence used in the illustration of FIG. 2 is fixed; however, the index k is changed to various values in order to check the values of the PAPR. This graph illustrates that differences equal to or greater than 3 dB can be caused in the values of the PAPR depending on the values of the index. The higher the PAPR a sequence has, the greater the peak power in comparison with the average power.
If a signal waveform of a preamble portion is not to be distorted upon the transmission of the preamble portion by a mobile terminal station, the back-off amount of the transmitting power of the transmission unit power amplifier of the mobile terminal station has to increase as the PAPR of the CAZAC sequence used as the preamble increases. As an amplifier operates with a greater back-off amount, the amplifier consumes more power. In other words, a transmission unit power amplifier presents a linear amplification characteristic with a smaller input power, whereas this amplification characteristic becomes non-linear with an input power around and greater than a certain input power value that is determined by the amplifier itself, and thereby the amplification ratio is saturated and decreases. When a signal using a CAZAC sequence with a high PAPR as a preamble is to be amplified, the difference between the average power and the peak power in this signal is great so that the peak power is not linearly amplified due to the saturation, resulting in a distortion in the signal if the position at which the average power is input is around the saturation region of the amplification characteristic of the amplifier. Accordingly, elimination of the distortion in a signal requires lowering of the average power of a signal input into the amplifier so that the power does not reach the region in which the amplification characteristic of the amplifier is saturated. A point including the average power of an input power or an output power in the amplification characteristic of an amplifier is referred to as an operating point, and an amount of a reduction from the output saturation point is referred to as a back-off amount.
Non-patent document 1 discloses specifications for the EUTRAN. Non-patent document 2 includes a description about CAZAC sequences such as a Zadoff-Chu sequence, etc.    Non-Patent Document 1:
3GPP TR25.814    Non-Patent Document 2:
3GPP TSG RAN1LTE Ad Hoc R1-061710